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PrinciplesofSoftwareConstruction: Objects,Design,andConcurrencyPart1:DesigningclassesAformaldesignprocess:DomainmodelingJoshBloch CharlieGarrod

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Administrivia

•  Homework3dueSundayat11:59p.m.•  Optionalreadingfortoday:

•  UMLandPatternsChapter17•  EffectiveJavaitems49,54,and69

–  RequiredreadingdueTuesday:•  UMLandPatternsChapters14,15,and16

•  MidtermexamnextThursday(February13th)–  ReviewsessionWednesday,February12th6-8pm,DHA302–  Practiceexamcomingthisweekend

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KeyconceptsfromTuesday

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KeyconceptsfromTuesday

•  Moredesignpatternsforreuse–  Templatemethodpattern–  Iteratorpattern–  Decoratorpattern

•  Designprinciples:heuristicstoachievedesigngoals

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Adesignprincipleforreuse:lowcoupling

•  Eachcomponentshoulddependonasfewothercomponentsaspossible

•  Benefitsoflowcoupling:–  Enhancesunderstandability–  Reducescostofchange–  Easesreuse

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Representationalgap

•  Real-worldconcepts:

•  Softwareconcepts:

PineTreeage height

harvest()

Forest -trees

…

Ranger …

surveyForest(…)

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Arelateddesignprinciple:highcohesion

•  Eachcomponentshouldhaveasmallsetofclosely-relatedresponsibilities

•  Benefits:–  Facilitatesunderstandability–  Facilitatesreuse–  Easesmaintenance

PineTreeage height

harvest()

Forest -trees

…

Ranger …

surveyForest(…)

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Problem

Space

Domain Model

Solution Space

Object Model

Our path toward a more formal design process

•  Real-worldconcepts•  Requirements,concepts•  Relationshipsamongconcepts•  Solvingaproblem•  Buildingavocabulary

•  Systemimplementation•  Classes,objects•  Referencesamongobjectsand

inheritancehierarchies•  Computingaresult•  Findingasolution

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Today

•  Interactiondiagrams:tovisualizedynamicbehavior•  Understandingadesignproblem:Object-orientedanalysis

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Visualizingdynamicbehavior:Interactiondiagrams

•  Aninteractiondiagramisapicturethatshows,forasinglescenarioofuse,theeventsthatoccuracrossthesystem’sboundaryorbetweensubsystems

•  Clarifiesinteractions:–  Betweentheprogramanditsenvironment–  Betweenmajorpartsoftheprogram

•  Forthiscourse,youshouldknowUMLsequencediagrams

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Constructingasequencediagram

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Anexamplesequencediagram

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(Sequencediagramwithnotationannotations)

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DrawasequencediagramforacalltoLoggingList.add:

publicclassLoggingList<E>implementsList<E>{privatefinalList<E>list;publicLoggingList<E>(List<E>list){this.list=list;}publicbooleanadd(Ee){System.out.println("Adding"+e);returnlist.add(e);}publicEremove(intindex){System.out.println("Removingat"+index);returnlist.remove(index);}…

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Today

•  Interactiondiagrams:tovisualizedynamicbehavior•  Understandingadesignproblem:Object-orientedanalysis

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Ahigh-levelsoftwaredesignprocess

•  Projectinception•  Gatherrequirements•  Defineactors,andusecases•  Model/diagramtheproblem,defineobjects•  Definesystembehaviors•  Assignobjectresponsibilities•  Defineobjectinteractions•  Model/diagramapotentialsolution•  Implementandtestthesolution•  Maintenance,evolution,…

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…

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Artifactsofthisdesignprocess

•  Model/diagramtheproblem,defineobjects–  Domainmodel(a.k.a.conceptualmodel)

•  Definesystembehaviors–  Systemsequencediagram–  Systembehavioralcontracts

•  Assignobjectresponsibilities,defineinteractions–  Objectinteractiondiagrams

•  Model/diagramapotentialsolution–  Objectmodel

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Artifactsofthisdesignprocess

•  Model/diagramtheproblem,defineobjects–  Domainmodel(a.k.a.conceptualmodel)

•  Definesystembehaviors–  Systemsequencediagram–  Systembehavioralcontracts

•  Assignobjectresponsibilities,defineinteractions–  Objectinteractiondiagrams

•  Model/diagramapotentialsolution–  Objectmodel

Today:understandingtheproblem

Definingasolution

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Inputtothedesignprocess:Requirementsandusecases

•  Typicallyprose:

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Modelingaproblemdomain

•  Identifykeyconceptsofthedomaindescription–  Identifynouns,verbs,andrelationshipsbetweenconcepts–  Avoidnon-specificvocabulary,e.g."system"–  Distinguishoperationsandconcepts–  Brainstormwithadomainexpert

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Modelingaproblemdomain

•  Identifykeyconceptsofthedomaindescription–  Identifynouns,verbs,andrelationshipsbetweenconcepts–  Avoidnon-specificvocabulary,e.g."system"–  Distinguishoperationsandconcepts–  Brainstormwithadomainexpert

•  VisualizeasaUMLclassdiagram,adomainmodel–  Showclassandattributeconcepts

•  Real-worldconceptsonly•  Nooperations/methods•  Distinguishclassconceptsfromattributeconcepts

–  Showrelationshipsandcardinalities

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Buildingadomainmodelforalibrarysystem

Apubliclibrarytypicallystoresacollectionofbooks,movies,orotherlibraryitemsavailabletobeborrowedbypeoplelivinginacommunity.Eachlibrarymembertypicallyhasalibraryaccountandalibrarycardwiththeaccount’sIDnumber,whichshecanusetoidentifyherselftothelibrary.Amember’slibraryaccountrecordswhichitemsthememberhasborrowedandtheduedateforeachborroweditem.Eachtypeofitemhasadefaultrentalperiod,whichdeterminestheitem’sduedatewhentheitemisborrowed.Ifamemberreturnsanitemaftertheitem’sduedate,thememberowesalatefeespecificforthatitem,anamountofmoneyrecordedinthemember’slibraryaccount.

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Buildingadomainmodelforalibrarysystem

Apubliclibrarytypicallystoresacollectionofbooks,movies,orotherlibraryitemsavailabletobeborrowedbypeoplelivinginacommunity.Eachlibrarymembertypicallyhasalibraryaccountandalibrarycardwiththeaccount’sIDnumber,whichshecanusetoidentifyherselftothelibrary.Amember’slibraryaccountrecordswhichitemsthememberhasborrowedandtheduedateforeachborroweditem.Eachtypeofitemhasadefaultrentalperiod,whichdeterminestheitem’sduedatewhentheitemisborrowed.Ifamemberreturnsanitemaftertheitem’sduedate,thememberowesalatefeespecificforthatitem,anamountofmoneyrecordedinthemember’slibraryaccount.

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Onedomainmodelforthelibrarysystem

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Notesonthelibrarydomainmodel

•  Allconceptsareaccessibletoanon-programmer•  TheUMLissomewhatinformal

–  Relationshipsareoftendescribedwithwords•  Real-world"is-a"relationshipsareappropriateforadomainmodel•  Real-wordabstractionsareappropriateforadomainmodel•  Iterationisimportant

–  Thisexampleisafirstdraft.Someterms(e.g.Itemvs.LibraryItem,Accountvs.LibraryAccount)wouldlikelyberevisedinarealdesign.

•  Aggregatetypesareusuallymodeledasclasses•  Primitivetypes(numbers,strings)areusuallymodeledasattributes

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BuildadomainmodelforHomework2

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PossibledomainmodelsforHomework2

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Understandingsystembehaviorwithsequencediagrams

•  Asystemsequencediagramisamodelthatshows,foronescenarioofuse,thesequenceofeventsthatoccuronthesystem’sboundary

•  Designgoal:Identifyanddefinetheinterfaceofthesystem–  System-levelcomponentsonly:e.g.,Auserandtheoverallsystem

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Understandingsystembehaviorwithsequencediagrams

•  Asystemsequencediagramisamodelthatshows,foronescenarioofuse,thesequenceofeventsthatoccuronthesystem’sboundary

•  Designgoal:Identifyanddefinetheinterfaceofthesystem–  System-levelcomponentsonly:e.g.,Auserandtheoverallsystem

•  Input:Domaindescriptionandoneusecase•  Output:Asequencediagramofsystem-leveloperations

–  Includeonlydomain-levelconceptsandoperations

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Onesequencediagramforthelibrarysystem

Usecasescenario:Alibrarymembershouldbeabletouseherlibrarycardtologinatalibrarysystemkioskandborrowabook.Afterconfirmingthatthememberhasnounpaidlatefees,thelibrarysystemshoulddeterminethebook’sduedatebyaddingitsrentalperiodtothecurrentday,andrecordthebookanditsduedateasaborrowediteminthemember’slibraryaccount.

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Formalizesystembehaviorwithbehavioralcontracts

•  Asystembehavioralcontractdescribesthepre-conditionsandpost-conditionsforsomeoperationidentifiedinthesystemsequencediagrams–  System-leveltextualspecifications,likesoftwarespecifications

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Asystembehavioralcontractforthelibrarysystem

Operation: borrow(item)Pre-conditions: Librarymemberhasalreadyloggedintothesystem.

Itemisnotcurrentlyborrowedbyanothermember.Post-conditions: Logged-inmember'saccountrecordsthenewly-borrowed

item,orthememberiswarnedshehasanoutstandinglatefee. Thenewly-borroweditemcontainsafutureduedate, computedastheitem'srentalperiodplusthecurrentdate.

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Distinguishingdomainvs.implementationconcepts

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Distinguishingdomainvs.implementationconcepts

•  Domain-levelconcepts:–  Almostanythingwithareal-worldanalogue

•  Implementation-levelconcepts:–  Implementation-likemethodnames–  Programmingtypes–  Visibilitymodifiers–  Helpermethodsorclasses–  Artifactsofdesignpatterns

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Summary:Understandingtheproblemdomain

•  Knowyourtoolstobuilddomain-levelrepresentations–  Domainmodels–  Systemsequencediagrams–  Systembehavioralcontracts

•  Befastand(sometimes)loose–  Elideobvious(?)details–  Iterate,iterate,iterate,…

•  Getfeedbackfromdomainexperts–  Useonlydomain-levelconcepts